This invention is directed to a molding composition comprising a blend of a polyarylate, a polyester, and at least one thermoplastic polymer selected from the group consisting of an aromatic polycarbonate, a styrene resin, an alkyl acrylate resin, a polyurethane, a vinyl chloride polymer, a poly(aryl ether), a copolyetherester block polymer or a polyhydroxyether.
Polyarylates are aromatic polyesters derived from a dihydric phenol, particularly 2,2-bis-(4-hydroxyphenyl)propane (also identified as Bisphenol-A) and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids.
Polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They have a high continuous use temperature of about 130.degree. C., and good unnotched toughness, with a pendulum impact value of greater than 300 ft. lbs./in..sup.3. Additionally, polyarylates have inherent flammability and combustion resistance as well as good weatherability. The polyarylates have good melt stability at high temperatures and good color retention. They also have good processability which allows them to be molded into a variety of articles.
Polyarylates have been blended with other resin systems such as ABS resin (U.S. Pat. No. 3,792,118), polycarbonate resins (U.S. Pat. No. 3,792,115), polyurethane resins, methyl methacrylate resins, etc. The blending of resins is often used to cover a defect of one resin with another resin. For example, a resin having a low heat distortion temperature may be blended with a resin having a high heat distortion temperature. However, this method is generally accompanied by unfavorable effects on other properties such as deterioration of mechanical properties or an inferior surface of the resulting injection molded resin.
Specifically, when polyarylates are blended with resins such as thermoplastic polyurethanes, vinyl chloride polymers, methyl methacrylate resins, etc. several problems result. Polyarylates have high viscosities which require a high molding temperature which exceeds the temperature stability limit of the other resin which is blended with the polyarylate. Thus, when this blend is molded deterioration of the resin occurs. This is observed particularly with blends of polyarylate and vinyl chloride polymers. Additionally, when there is an extreme viscosity difference between the high viscosity polyarylate and the other polymer severe surface irregularities (e.g. jetting) are observed when these blends are injection molded. This is observed especially when polyarylate is blended with, for example, ABS resins and poly(methyl methacrylate) resins. Further, the extremely high viscosity of polyarylates prevents a uniform product from being obtained when the polyarylate is blended with a resin having a lower viscosity by conventional polymer mixing techniques, i.e., extrusion or Banbury type melt mixing. The non-uniform blend will not weather as well as a uniform blend of the resins nor will it have an acceptable balance of properties.
It has now been found that the addition of a polyester derived from an aliphatic or cycloaliphatic diol, or mixtures thereof, and an aromatic dicarboxylic acid to a blend of a polyarylate and at least one thermoplastic polymer selected from the group consisting of an aromatic polycarbonate, a styrene resin, an alkyl acrylate resin, a vinyl chloride polymer, a poly(aryl ether), a copolyetherester block polymer or a polyhydroxyether, produces a blend which has excellent surface appearance and can be easily molded without deterioration.
Additionally, it has been found that the addition of polyarylate to a blend of the polyester and the thermoplastic polymer improves the weatherability of the blend, i.e., the mechanical properties of the blends are retained after exposure to conditions of ultraviolet light and moisture. The presence of the polyester resin in the blend provides a uniform blend and the resins can be blended using conventional polymer mixing techniques. The presence of the polyester resin in the blend does not interfere with the improving effect the polyarylate has on the weatherability of the blend.
The blends of the present invention have excellent mechanical properties, better than the properties of a binary blend of polyarylate and a thermoplastic polymer. This is unexpected and allows heretofore unusable binary systems of polyarylate resin and thermoplastic polymer to be molded into articles.